Understanding the dynamics of secondary bile acid (SBA) formation in the gut by SBA-producing bacteria is important for host health, as SBAs have been shown to affect host pathophysiology and gut microbiota composition. However, our knowledge of SBA producers is limited in light of the diversity of gut microbes. Here, we isolated six novel SBA-producing bacteria from rat cecal contents, all of which were members of known species of gut microbes. Anaeros-tipes caccae DIO, Bacteroides nordii C5, Clostridioides difficile D7, and Clostridium cadaveris Gil were capable of oxidizing cholic acid and chenodeoxycholic acid into 7-oxo-derivatives with varying yields. B. nordii C5 and its type strain JCM 12987(T) had the highest molar yield, similar to 90%. Clostridium disporicum F4 and Clostridium subterminale C4 epimerized cholic acid into ursocholic acid with yields of similar to 85%; the corresponding type strains lacked epimerization activity. Further more, although not novel as an SBA producer, Clostridium scindens G10 that produced deoxycholic acid from cholic acid was isolated for the first time from rodents. These findings will contribute to elucidation of SBA formation in the gut. (C) 2017, The Society for Biotechnology, Japan. All rights reserved.